Transport of Powders through Rotary Kilns :

Experimental Study and Modelling

M. Debacq, D. Hartmann, J.L. Houzelot, D. Ablitzer

ECCE2,

October 5-7, 1999,

Montpellier, France.

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

OUTLINE

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

OUTLINE

During the nuclear fuel cycle ...

depleted UF6

U3O8

storagekiln A

enrichment

enriched UF6

UO2

nuclear

reactor

kiln B

Conversion Reactor

hydrolysis

VERTICAL

REACTOR

U3O8

or

UO2

H2O

N2

UF6

H2O, H2

H2O, HF,

H2, N2

Archimedes' screw

heating devicelifter

baffle (only in kiln B)

pyrohydrolysis

ROTARY KILNUO2F2

H2O, HF, H2

Cross section :

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

OUTLINE

Transverse Bed Motion

general view

cross section

rotary kilns without inner equipment

Transverse bed motion

varies with and material properties

Rolling bed regime

favourable for heat and mass transfer

Mean Residence Time and R.T.D.

Axial transport

essentially plug flow with low dispersion

Mean residence time = function of

design parameters (L, R, )

material properties (, dp)

operating parameters (Q, )1

?QL

rotary kilns without inner equipment

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

OUTLINE

R.T.D. Measurement

Isotopic tracer technique

square signal of more enriched UF6 into vertical reactor

gamma radiation measurement and

Parameters

feed rate Q

rotational speed

U3O8

or

UO2

H2O, H2

H2O, HF,

H2, N2

UO2F2

H2O,

HF, H2

H2O

N2

UF6

Despite production necessities, RTD measurements on both industrial kilns

UF6

injection

plug flow with low dispersion

rotary kiln outlet

rotary kiln inlet

0.45 % 235U

0.29 % 235U

kiln A kiln B

0.23 % 235U

4 % 235U

Comparison with Correlations from Literature

0 1Q

0 1Q

0.2 0.8Q

1.2 0.9Q

kg/h 895 508 712 888 889

rpm 2.1 2.1 2.1 2.7 1.6

experiment 19 37 23 16 26

Afacan 28 30 28 22 36

Das Gupta 25 28 26 20 31

Ronco 21 22 21 16 26

kg/h Q1 Q2 Q1 Q1 Q3

rpm 1 2 2 3 4

experiment 14.7 29.8 29.5 18.7 15.0

Afacan 13.8 30.2 29.3 18.6 14.1

Das Gupta 13.2 28.4 27.2 17.6 13.5

Ronco 13.5 30.3 29.5 18.5 13.5

kiln

Akiln

B

min

min

feed rate (UF6)

rotational speed

mean

residence

time

feed rate (UF6)

rotational speed

mean

residence

time

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

OUTLINE

Models for Bed Depth Profiles

Kramers then Afacan

Das Gupta

Ronco

boundary condition hL = hlifter

2/32

3v

R

h

R

h2

R4

tanQ3

cos

tan

dx

dh

2

2 2

3 / 22

v

3 2

1

2 h h1

R R2 h h h1 ln

hR R R1

Rdh tan 3 Q tan 2 h h

dx cos 4 R R R h h 2 h hcos 1 1

R R R R

75.03v

ZD

sinQ4'Ktan

dx

dh

L

0

v

L

0

dxQ

A

u

dx

Results

0

10

20

30

40

50

fill

ing

ra

tio

[%

] in

kil

n A

.

0

2

4

6

8

10

12

14

fill

ing

ra

tio

[%

] in

kil

n B

.

Afacan

Das Gupta

Ronco

kiln input kiln output

higher than usually estimated by industrial kiln operators

improve models to take lifters, perhaps baffles, into account

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

OUTLINE

kiln B

kiln A

Scale Models (Working at room temperature)

behaviour filmed through the glass end wall

( operating parameters)

image analysis proportion of powder lifted

influence of baffles on the axial motion

(in a one meter long model with a glass wall)

A segment of the corresponding industrial kiln

Powder Pouring from Lifters

Qualitative Observations

large amounts of powder hold

unload progressively

well-mixed state after only 2 or 3 revolutions

axial motion : powder retention above baffles (if filling ratio is high enough)

Typical filling of lifters :

efficient powder/gas contact

lifter position

po

wd

er

fra

cti

on

angle of

end of unloading :

140 °

Behaviour during Discharge

Identical for any :

rotational speed

filling ratio

powder

kiln

lifter angular position (towards horizontal)

fra

ctio

n o

f p

ow

de

r in

th

e lifte

r

(to

wa

rds to

tal m

od

el vo

lum

e)

Unloading law

0.00013 140

Will be used for

calculation of powder / gas contact area

conversion

heat transfer

geometrical description of the motion of a particle

bed depth profile

mean residence time

OUTLINE

Introduction

Previous studies

Axial transport in rotary kilns with inner equipment

R.T.D.

Bed depth profiles

Transverse motion in rotary kilns with inner equipment

Final remarks

Conclusions

Axial transport

plug flow with low dispersion

mean residence time kiln A

kiln B

3 models tested for bed depth profiles

Transverse motion of cohesive powders in rotary kilns with inner equipment

unique and simple unloading law

1.2 0.9

0.2 0.8

Q

Q

0.00013 140

Prospects

Experimental and theoretical studies

effect of gas countercurrent

effect of temperature

Global mathematical model of the kiln

hydrodynamics of cohesive powders in rotary kilns with inner equipment

mean residence time

bed depth profiles

use of powder size distribution to calculate : powder / gas contact area